Dietary Fiber from Tunisian Common Date Cultivars (Phoenix

Article pubs.acs.org/JAFC

Dietary Fiber from Tunisian Common Date Cultivars (Phoenix dactylifera L.): Chemical Composition, Functional Properties, and Antioxidant Capacity Abdessalem Mrabet,† Rocío Rodríguez-Arcos,§ Rafael Guillén-Bejarano,§ Nizar Chaira,† Ali Ferchichi,† and Ana Jiménez-Araujo*,§ †

Arid and Oases Cropping Laboratory, Arid Area Institute, Medenine 4119, Tunisia Instituto de la Grasa, Consejo Superior de Investigaciones Científicas (CSIC), Avenida Padre García Tejero 4, Sevilla 41012, Spain

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ABSTRACT: The dietary fibers (DF) of 10 date varieties from Tunisian oases have been investigated. Further knowledge on the content, composition, and technological applications of those fibers could support their genetic variability and promote the socioeconomical development of growing areas. The composition, water- and oil-holding capacities, solubility, and antiradical activity have been determined. The DF content ranged from 4.7% (Matteta, Rochdi) to >7% (Deglé Nour, Garen Gaze, Smeti). Composition varied significantly among cultivars, and the results evidenced that uronic acids and lignin determine to a great extent the organoleptic quality of dates. Many of the varieties that have been studied (Garen Gaze, Matteta, Kenta, Rochdi, Mermella, Korkobbi, Eguwa) were selected because of great interest from technological and functional points of view. Among their physicochemical characteristics, these samples presented water- and oil-holding capacities of higher than 17 and 4 mL/g fiber, respectively, which make them suitable for use as additives in fiber-enriched foods. Also, DF of Garen Gaze, Smeti, Mermella, and Eguwa had a high antiradical capacity (>230 Trolox equiv/kg fiber). It was concluded that some of these varieties could be grown as potential sources of DF, which could be included in the formulation of fiber- and antioxidant-enriched foods. KEYWORDS: dietary fiber, date palm fruits, chemical composition, functional characteristics, antioxidant capacity

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INTRODUCTION The date (Phoenix dactylifera L.) is an important crop in arid and semiarid regions of the world. Nearly 2000 cultivars of date palm are known in the world, but only some of them are evaluated for their performance and fruit quality. Date production has tripled from 1 915 615 tons in 1975 to 6 002 040 tons in 2005.1 In Tunisia, the mean annual production of date fruits has remarkably improved and reached an average of 120 000 tons/year, dominated by the Deglé Nour variety (60% of total production), which has a highly appreciated sensory quality and high marketing value. This progress in production, at the national and international scales, is unfortunately accompanied by a considerable increased loss in secondary or common-variety dates (approximately 30 000 tons for Tunisia and 2 000 000 tons worldwide). These dates are generally rejected or in some limited cases used for animal feed. It is by this selective orientation that we are currently witnessing a progressive disappearance of secondary cultivars and therefore a reduction in genetic variability. Among the threatened cultivars are those of the coastline oasis of Gabès. To fight this ecological and economic problem, several studies on the valorization of common dates have been conducted. The literature mentions certain technological transformations, for example, the production of jams, frosts, juices, and syrups of dates.2,3 The chemical composition of the date shows that the flesh is an important source of sugar (81−88%, mainly fructose, glucose, and sucrose), dietary fiber (DF, 5−8.5%), and small amounts of protein, fat, ash, and polyphenol.4,5 Thus, dates provide a good source of rapid energy (sugars) and good nutritional value, based on their DF contents. © 2012 American Chemical Society

The demand for byproducts from fruits and vegetables as sources of DF has been increasing because these sources offer higher nutritional quality, higher amounts of total and soluble fiber, lower caloric content, stronger antioxidant capacity, water- and oil-holding capacities, and colonic fermentability, as well as a lower phytic acid content6,7 than cereal byproducts. DF plays an important role in human health and has shown beneficial effects in the prevention of several diseases, such as cardiovascular diseases, diverticulosis, coronary heart disease, constipation, irritable colon, colon cancer, atherosclerosis, obesity, and diabetes.5,7,8 To date, limited data are available regarding the compositional and functional characteristics of common dates grown in Tunisia. 9 The common date may possess high-value components that may be used in value-added applications, including their use as functional foods and ingredients in nutraceuticals.5,10 The objective of this study was to isolate common date dietary fibers and evaluate their chemical composition and functional properties. This is the first time that a complete study on the chemical composition of date DF has been done. Nowadays, the antioxidant capacity is one of the most valuable among the functional properties. We have studied this capacity in the DF of common dates, especially the one linked to fiber. This is the characteristic that could ultimately promote the use Received: Revised: Accepted: Published: 3658

September 6, 2011 March 9, 2012 March 23, 2012 March 23, 2012 dx.doi.org/10.1021/jf2035934 | J. Agric. Food Chem. 2012, 60, 3658−3664

Journal of Agricultural and Food Chemistry

Article

of dates and their corresponding fibers as “antioxidant fibers” in the market of healthy ingredients for functional food formulation.

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was quantified from the trifluoroacetic acid-insoluble residue after 72% sulfuric acid hydrolysis according to the anthrone method. Water-Holding Capacity (WHC). The WHC was determined using the method described by Jiménez et al.14 Samples (250 mg × 3) were suspended in 15 mL of water. After 24 h of stirring at room temperature, the suspension was centrifuged at 14000g for 1 h. Supernatants were carefully eliminated, and the hydrated fibers were weighed. WHC was expressed as milliliters of water per gram of fiber. Hydrated pellets were freeze-dried, and their solubility in water was determined by the difference in weight between before and after the WHC assay, which was expressed as a percent. Oil-Holding Capacity (OHC). The OHC was determined using the method described by Jimenez et al.14 OHC was measured by adding 15 mL of sunflower oil (1.0054 g/mL density) to a concentrate of date fibers (250 mg × 3) in a 50 mL centrifuge tube. The content was stirred for 24 h at room temperature; then, the tubes were centrifuged at 14000g for 1 h. Supernatants were carefully eliminated, and the oil-embedded fibers were weighed. Oil-holding capacity was expressed as milliliters of oil per gram of fiber. Extraction of Soluble Phenols. Ten grams (by duplicate) of fresh pulp was homogenized in an Ultraturax at top speed for 1 min with 50 mL of a 70:30 mixture of acetone/water. The suspension was extracted over 30 min by sonication in an ice bath. After filtration, the slurry was extracted again under the same conditions. Both filtrates were collected. The solvent was evaporated under vacuum, and the extracts were redissolved in 10 mL of the same solvent. These concentrated extracts were used to determine soluble phenols and soluble antiradical activity. The dry pellet was used to determine the antioxidant activity of the fiber portion. The yield of this extraction step was used to generate results for antioxidant activity on a fresh weight (FW) basis. Determination of Soluble Phenols. The total polyphenol content was quantified for each date extract according to the Folin− Ciocalteu spectrophotometric method, using gallic acid as a reference standard. Aliquots of 0.2 mL of each sample were dosified in triplicate, and 0.5 mL of Folin−Ciocalteu phenol reagent (0.2 M) was added to each tube and mixed. Afterward, 0.4 mL of Na2CO3 (75 g/L) was added and mixed well. A microplate reader was set at 630 nm, and the absorbance was measured after 60 min. Results are expressed as gallic acid (GA) equivalents (mg/100 g FW). Determination of the Antiradical Activity. Soluble antioxidant activity was determined from the soluble phenol extracts by the DPPH• method.15 Fiber antioxidant activity was evaluated as described by Fuentes-Alventosa et al.16 Between 3 and 20 mg of soluble phenol-free fibers was transferred to an eppendorf tube (for weights of 95%. The soluble and insoluble nature of DF involves differences in their technological functionality and physiological effects. Insoluble fibers are characterized by their porosity, their low density, and their ability to increase fecal bulk and decrease intestinal transit. On the basis of these values, date fiber could be used as a modifier of viscosity and texture of formulated products in addition to promoting the decrease in calories that this addition could imply. The OHC results are presented in Table 3. Eguwa, Garen Gaze, and Korkobbi had the highest capacity (>4 mL/g fiber), which was much lower than that found in a previously cited work: approximately 9.7 g/g fiber.9 These authors applied a force of 1500g to determinate the OHC instead of the 14000g we applied to our samples. The values found in the literature for other byproducts, for example, 0.6−1.8 g/g for apple pomace and citrus peel,6 were much lower than those we reported for date fiber. Thus, the use of this fiber may be appropriate in products that require emulsifying properties. Polyphenol Content and Antioxidant Activity of Date Fruits. The polyphenol content and antioxidant activity of both fractions, soluble and linked to fiber, are presented in Table 4. Deglé Nour was the variety that had the highest soluble polyphenol content (221 mg GA/100 g FW). From the coastal growth zone, Bouhattam, Korkobbi, and Eguwa were richer in these compounds than the other varieties, followed by Kenta, Limsi, Matteta, Garen Gaze, Smeti, Mermella, and Rochdi. Besides variety and growth zone, the soluble polyphenol content also depends on date humidity. There are important differences between soft, semidry, and dry dates. Biglari et al.,28,29 who worked with Iranian dates, found that the polyphenol content in soft dates varies from 2 to 4 mg GA/

range of lignin content from other products was wide, varying from 6% for peach pulp27 to 32% for cocoa bean husks.24 The neutral sugar composition of DF was also studied (Table 2). Xylose was the main sugar, accounting for nearly 50% of the molar percentage. Other sugars of interest were arabinose (between 17 and 22%) and galactose (8−16%). Mannose and glucose were near 5%, and rhamnose and fucose were found in lower amounts. Elleuch et al.9 reported higher percentages of rhamnose and galactose than those found in our varieties. These differences could be related to higher amounts of uronic acids also quantified in those samples. Moreover, pectins, xylans, and arabinoxylans are the main noncellulosic polysaccharides present in date DF, representing approximately 75% of the total neutral sugars. This composition is very similar to that of other lignocellulosic agricultural byproduct. Some authors have compared it to hardwoods and straws rather than softwoods, which contain large amounts of mannose.22 Water- and Oil-Holding Capacities and Solubility of Date Fiber. The results obtained for WHC, solubility, and OHC are presented in Table 3. WHC is an important property of DF from both physiological and technological points of view. The WHC of date DF ranged from 12.65 to 17.22 mL water/g fiber. The Kenta variety had the highest value, and Deglé Nour, Mermella, Rochdi, and Matteta exhibited nonstatistical differences with respect to WHC. Other authors9 reported a capacity Table 3. Functional Properties of Date Fruit Fiber from Different Cultivarsa cultivar Rochdi Matteta Korkobbi Eguwa Bouhattam Mermella Limsi Kenta Deglé Nour Garen Gaze Smeti

WHC (mL water/g fiber) 16.23 15.96 14.77 12.65 14.56 16.45 15.63 17.22 16.54 13.01 13.48

± ± ± ± ± ± ± ± ± ± ±

0.24 def 1.25 def 0.63 bcd 0.75 a 0.41 bc 0.95 ef 0.49 cde 0.92 f 1.47 ef 0.64 a 1.05 ab

% SOL 0.23 0.15 0.15 0.10 0.18 0.20 0.13 0.14 0.17 0.11 0.23

± ± ± ± ± ± ± ± ± ± ±

0.01 f 0.01 c 0.00 c 0.01 a 0.00 d 0.01 e 0.02 b 0.01 bc 0.01 d 0.01 a 0.00 f

OHC (mL oil/g fiber) 3.84 3.53 4.21 4.46 3.83 4.11 3.72 3.38 3.64 4.25 3.01

± ± ± ± ± ± ± ± ± ± ±

0.29 cd 0.07 bc 0.08 ef 0.11 f 0.38 cd 0.18 de 0.16 bc 0.09 b 0.28 bc 0.14 ef 0.31 a

a

Values are the means of triplicate assays. Means bearing the same letter are not significantly different at the 5% level as determined by the Duncan multiple-range test. WHC, water-holding capacity; SOL, solubility; OHC, oil-holding capacity. 3661

dx.doi.org/10.1021/jf2035934 | J. Agric. Food Chem. 2012, 60, 3658−3664


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Table 4. Phenol Contents and Antioxidant Activities of Different Date Fruit Cultivarsa soluble fraction cultivar Rochdi Matteta Korkobbi Eguwa Bouhattam Mermella Limsi Kenta Deglé Nour Garen Gaze Smeti

phenol content (mg GA/100 g FW) 28.96 84.72 145.01 146.65 164.50 41.20 94.48 96.62 221.32 79.40 66.22

± ± ± ± ± ± ± ± ± ± ±

2.30 a 7.17 cd 8.17 f 13.41 f 13.40 g 3.79 a 11.70 de 9.57 e 18.35 h 6.60 c 3.23 b

fiber fraction

antiradical activity (mmol Trolox/kg FW) 3.07 11.58 28.68 9.95 24.60 3.18 8.82 9.66 50.82 12.00 8.04

± ± ± ± ± ± ± ± ± ± ±

0.63 a 0.98 cd 5.32 f 2.13 bcd 1.51 e 0.51 a 1.25 b 1.13 bc 4.80 g 2.81 d 0.50 b

p-coumaric acid (mg/100 g)

vanillin (mg/100 g) 39.57 10.39 42.99 50.74 25.83 23.37 25.85 25.67 11.90 25.23 17.92

± ± ± ± ± ± ± ± ± ± ±

7.63 0.95 1.08 7.48 1.81 0.96 1.46 0.15 0.43 0.97 0.83

26.98 17.39 5.62 8.64 6.52 24.10 15.90 12.01 8.11 6.07 13.89

± ± ± ± ± ± ± ± ± ± ±

11.11 1.33 2.23 0.99 1.04 2.37 0.12 0.52 0.07 1.32 1.51

ferulic acid (mg/100 g) 49.27 56.35 21.25 35.51 31.43 28.81 45.40 35.33 14.35 33.16 27.39

± ± ± ± ± ± ± ± ± ± ±

7.64 3.04 0.82 1.79 1.38 1.53 0.46 0.88 0.63 0.22 0.29

total phenols (mg/100 g) 115.82 84.12 69.86 94.88 63.77 76.28 87.15 73.01 34.36 72.12 59.19

± ± ± ± ± ± ± ± ± ± ±

11.10 h 0.77 ef 1.97 cd 8.28 g 4.23 bc 4.86 de 0.88 fg 0.51 cd 0.13 a 1.95 cd 2.63 b

antiradical activity (mmol Trolox/kg fiber) 104.32 38.75 73.98 232.30 68.74 278.68 71.94 79.45 203.21 230.06 301.36

± ± ± ± ± ± ± ± ± ± ±

2.71 d 4.13 a 0.67 bc 2.39 f 0.23 b 3.03 g 2.07 b 1.50 c 0.81 e 6.25 f 10.23 h

a

Values are the means of at least duplicate assays. Means bearing the same letter are not significantly different at the 5% level as determined by the Duncan multiple-range test.

compounds could confer additional antioxidant activity that was not measured with the soluble extracts. In Figure 1, the

100 g FW, that in semidry from 4 to 6 mg GA/100 g FW, and that in dry 141 mg GA/100 g FW. Similar results were published for Algerian soft dates,30 between 2 and 8 mg GA/ 100 g FW, and for Omani dry dates,5 from 172 to 246 mg GA/ 100 g FW. Chaira et al.,31 who worked with Tunisian dates from some of the varieties presented in this study, found a smaller soluble polyphenol content (22−110 mg/100 g FW), most likely due to differences in the extraction and quantification methods. In agreement with our work, they found that Korkobbi, Bouhattam, and Kenta had the highest polyphenol content (they did not analyze the Eguwa cultivar). The antiradical activity of the soluble fraction was studied, the results of which are presented in Table 4. As was the case with the polyphenol content, Deglé Nour had the highest activity (50 mmol Trolox/kg FW). Korkobbi and Bouhattam cultivars had nearly half that of the Deglé Nour activity (28.68 and 24.60 mmol Trolox/kg FW, respectively). The lowest values were those found for Rochdi and Mermella (3.07 and 3.18 mmol Trolox/kg FW, respectively). These results are also in agreement with those published by Chaira et al.31 Although sugars and other compounds could have slight interference in the Folin−Ciocalteu quantification method, there was good correlation between the soluble polyphenol content and antiradical activity (R = 0.8451), as has been reported by other authors.28,30,31 In this work, we also studied the phenol composition and antiradical activity of the insoluble fraction. This fraction could be very important with regard to the total antioxidant activity of fruits and vegetables because, as Saura-Calixto32 suggested, the transportation of antioxidants through the gastrointestinal tract is an essential physiological function of DF. This aspect has received very little attention so far. Bound phenolics were extracted from all samples and identified and quantified by HPLC. The results are presented in Table 4. Ferulic acid was the most abundant simple phenol in all of the samples, except for Korkobbi and Eguwa, in which vanillin was the major phenol. Ferulic acid ranged from 14.35 to 56.35 mg/100 g fiber in the Matteta and Deglé Nour varieties, respectively. The other phenolic acid identified, p-coumaric acid, was the minor component in almost all of the samples. The total amount and composition varied significantly among the samples, as reported by Al-Farsi et al.,18 who worked with Omani dates. Because we found that date pulp contained an interesting amount of phenols linked to fiber, we considered that these

Figure 1. Dose−response lines of antiradical capacity of date fibers from different cultivars. Antiradical capacity is expressed as percent DPPH• remaining in solution after 30 min of reaction. Each individual point in the graph is the average value of three replicates. DN, Deglé Nour; Bh, Bouhattam; GG, Garen Gaze; L, Limsi; Mt, Matteta; Kt, Kenta; Rch, Rochdi; St, Smiti; Mr, Mermella; Kor, Korkobbi; Eg, Eguwa.

antiradical activity of date fiber is presented. In this figure, the higher scope in the regression line indicates the higher antioxidant activity. The assayed fibers were soluble phenolfree, so their activities should be due exclusively to compounds present in the fiber fraction. There was a group of five varieties (Deglé Nour, Garen Gaze, Smeti, Mermella, and Eguwa) that had the highest activities. Bouhattam, Limsi, Kenta, Rochdi, and Korkobbi had an intermediate level of free radical scavenging activity, Matteta being the least active variety. In Table 4, these levels of activity are expressed as millimoles of Trolox per kilogram of fiber. The variety with the highest activity linked to fiber was Smeti; the order of the remaining samples with regard to this activity was as follows: Smeti > Mermella > Garen Gaze = Eguwa > Deglé Nour ≫ Rochdi > Kenta > Korkobbi > Limsi = Bouhattam ≫ Matteta. In the fiber fraction, there was no correlation between the polyphenol content and antiradical activity. In prepared food, which undergoes a thermal process, a nonenzymatic browning reaction takes place, leading to the formation of a complex series of compounds called Maillard 3662



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Funding

reaction products. The antioxidant activity of these compounds has been reported in several studies.33 In date palm fruits, this reaction was mentioned by Barneveld.34 In our work, these compounds could also contribute to the antioxidant activity of the fiber fraction; thus, a more detailed study on date Maillard reaction products should be conducted. The most interesting thing is that four Tunisian varieties had high antiradical activity, even higher than that of the Deglé Nour variety. The fiber from these date varieties could be considered “antioxidant DF”, as defined by Saura-Calixto,35 because they have a free radical scavenging capacity of >50 mg of vitamin E (measured by the DPPH• method), and this activity is derived from natural components of the material. Figure 2 shows the total

We are grateful to the Institut des Regions Arides (Medenine, Tunisia) for the financial support of travel expenses of A.M. for his stay with the Instituto de la Grasa (Sevilla, Spain). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS USED DF, dietary fiber; MES, 4-morpholineethanesulfonic acid; Tris, tris(hydroxymethyl)aminomethane; DPPH, 2,2-diphenyl-1-picrylhydrazyl free radical; GA, gallic acid; FW, fresh weight.

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Figure 2. Total antiradical activity expressed as millimoles of Trolox per kilogram of fresh weight of date fruits from different cultivars. DN, Deglé Nour; Bh, Bouhattam; GG, Garen Gaze; L, Limsi; Mt, Matteta; Kt, Kenta; Rch, Rochdi; St, Smiti; Mr, Mermella; Kor, Korkobbi; Eg, Eguwa.

antioxidant activity expressed on a fresh weight basis. It is clear that Deglé Nour is the best variety, with the highest activities (soluble and linked to fiber). However, fiber from Garen Gaze, Smeti, Mermella, and Eguwa cultivars had an activity similar to or even higher than that of Deglé Nour. These are the Tunisian secondary varieties that could be promising from a technological point of view. In summary, the DF contents of 10 Tunisian date varieties from Gabès coastal oases have been studied for the first time and compared to that of Deglé Nour, the most commercially accepted variety. Although Tunisian secondary varieties have similar DF content and composition, they are not very suitable for human consumption. Therefore, our study also focused on possible technological applications. The most important characteristic supporting the use of some of these varieties as food ingredients is their antioxidant activity. The level of activity found in the fiber of Garen Gaze, Smeti, Mermella, and Eguwa varieties makes them valuable as potential sources of antioxidant fiber. Further studies on the responsible compounds (maybe Maillard reaction products) for this characteristic are needed, as is a technological approach to obtaining this antioxidant DF from date fruit. The use of these secondary varieties in the food industry as healthy ingredients could help in the fight against the reduction in vegetal genetic variability. In addition, the growth of these cultivars for technological purposes may play an important role in the economic and social level of the people from this developing region.

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Dietary Fiber from Tunisian Common Date Cultivars (Phoenix

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